1. Field of the Invention
This invention relates to fatigue monitors and more particularly to a novel and highly effective device for detecting fatigue of a monitored structure due to flexure and to a method of making the device. The invention is applicable particularly to the detection of metal fatigue but is applicable also to the detection of fatigue in other structural materials.
2. Description of the Prior Art
Maurice A. Brull U.S. Pat. Nos. 4,590,804 and 4,639,997, both assigned to the assignee of the present invention, disclose respectively a device for and method of monitoring fatigue life. The patents illustrate what are called coupons, otherwise known as fatigue fuses, arranged in a row and each having different notch patterns ranging from the mildest notch pattern to the most aggressive notch pattern. The coupons will fail because of fatigue in a prescribed sequence. The number of load cycles to failure as a function of stress amplitude for the different coupons can be plotted, and as each coupon fails, it gives an indication of the remaining life expectancy of the monitored structure.
More particularly, each of the coupons includes a special notch pattern. The coupon axis must be oriented along a suitably chosen direction. The notch pattern of each of the coupons produces a stress field which varies in intensity from relatively mild to very severe. The severity of the local stress field is controlled by the geometry of the notch pattern. Smooth geometries produce a mild stress concentration, while geometric discontinuities produce severe stresses. In this manner, when all of the coupons are subjected to the same stress history, it will result in the development of different stress concentrations in the region of the notch tips of each coupon, so that each coupon will have a different fatigue life. Moreover, if the stress history of the coupons is the same as that of the monitored structure, the fatigue life of each coupon will be a different percentage of the fatigue life of the monitored structure.
In accordance with the disclosures of the patents, the coupons are secured to the monitored structure by pins, adhesive or welding so that, ideally, all of the coupons experience the same strain history as the monitored structure.